Bringing the Web Up to Speed with

WebAssembly

Matthew Furlong, Drew Davis, Ayush Goel, Umang Lathia

An Open Standard

An Open Standard

These 4 browsers have a 90 percent

market share!

Outline1. Introduction and Motivation2. Overview and Execution3. Validation4. Binary Format & Embedding5. Evaluation

The Web● “The most ubiquitous

application platform ever.”

The Web● “The most ubiquitous

application platform ever.”● Yet Javascript is the only

natively supported programming language on the web...

Away from Javascript...● Web applications are more demanding than ever

○ 3D Visualization○ Audio and Video software○ Games

● Many developers don’t want to use Javascript

... and Onto WebAssembly!● A low-level, language independent bytecode for the Web

... and Onto WebAssembly!● A low-level, language independent bytecode for the Web● Goals

○ Safe○ Fast○ Portable○ Compact

Previous Work on Bytecode for the Web● Microsoft’s ActiveX● Native Client and Portable Native Client● asm.js

Previous Work on Bytecode for the Web● Microsoft’s ActiveX● Native Client and Portable Native Client● asm.js

WebAssembly is the first solution for low-level code on the Web that provides safety, speed, portability, and small code

sizes.

Outline1. Introduction and Motivation2. Overview and Execution3. Validation4. Binary Format & Embedding5. Evaluation

Overview● A binary code format, not a language ● Basic language features

○ Modules○ Functions○ Instructions○ Traps○ ….

Overview● New Language features

○ Linear memory ( also known as flat memory)○ Endiannes

■ Little endian○ Structured Control Flow

■ Eliminates problems caused by simply jumps■ Blocks execute like function calls

○ Function calls

Overview● Determinism

○ Design semantics tries to minimize non determinism due to corner cases.

○ Implementation dependent behavior ■ NaNs■ Resource Exhaustion■ Host Functions

Execution● Uses a global store object ( like Windows in Browsers)

● Stores and Runtime objects representation

● Reduction Rules

Outline1. Introduction and Motivation2. Overview and Execution3. Validation4. Binary Format & Embedding5. Evaluation

Validation

Untrusted Server

Local Browser

WebAssembly Code

Validation● Defined as a simple type system● Efficiently checkable in a single linear pass

Validation● Typing Rules

○ Ensure that the types for every instruction sequence are correct

● Soundness○ Typing rules cover all possible states (no undefined

behavior)■ Guarantees memory safety and inaccessibility of

code addresses or call stack

Outline1. Introduction and Motivation2. Overview and Execution3. Validation4. Binary Format & Embedding5. Evaluation

Binary Format● Code transmitted across web as a binary encoding

○ Binary code organized by entities■ Streaming compilation ■ Parallelized compilation

○ Instructions - one-byte opcodes○ Integral numbers - LEB128 format

Embedding● WebAssembly is designed to be embedded into an

execution environment● Therefore, does not define:

○ How programs are loaded into execution environment○ How I/O is performed

Outline1. Introduction and Motivation2. Overview and Execution3. Validation4. Binary Format & Embedding5. Evaluation

Implementation● Lots of different JavaScript engines

○ V8 (Chrome), SpiderMonkey (Mozilla), Chakra (Edge)

● Developed independent implementations for each browser○ On-the-fly validation (as fast as 1 GB/s)○ SSA (V8 and SpiderMonkey) → direct-to-SSA in a single pass

● Other Optimizations○ Bounds Check - Constant-fold memsize - offset ○ Parallel Compilation (5-6x improvement)○ Compiled code caching (memoization)

Measurements Execution time of PolyBenchC benchmarks on Webassembly normalized to native code

Measurements Execution time of PolyBenchC benchmarks on Webassembly normalized to native code

7 benchmarks within 10% of nativeAlmost all within 2x of native

Measurements Execution time of PolyBenchC benchmarks on Webassembly normalized to native code

33.7% faster than asm.js (validation

much faster)

Measurements Binary size of WebAssembly in comparison to asm.js and native code

Measurements Binary size of WebAssembly in comparison to asm.js and native code

62.5% size of asm.js85.3% size of native x86

Measurements Binary size of WebAssembly in comparison to asm.js and native code

Evaluation● Strength

○ Ability to write in any language

○ Faster compilation

○ Compact

○ Fast

● Weaknesses○ Separate compiler to port each language to WebAssembly

Road Map● MVP Completed (3 years ago)● Features in process:

○ Exception handling○ Threads○ Garbage Collection○ Single Instruction Multiple Data instructions○ Tail Calls

https://webassembly.org/docs/future-features/

Community & Current Updates

Questions?

Appendix A - Why is WebAssembly faster than asm.js?

● Startup○ Smaller to download, faster to parse

● CPU features

○ asm.js doesn’t have access to CPU features -- slower

●